Materials: Coating Turns Ordinary Glass into Super Glass

A new transparent, bioinspired coating makes ordinary glass tough, self-cleaning and incredibly slippery – according to a team from the Wyss Institute for Biologically Inspired Engineering at Harvard University and Harvard School of Engineering.

The new coating could be used to create durable, scratch-resistant lenses for eyeglasses, self-cleaning windows, improved solar panels and new medical diagnostic devices, said principal investigator Dr. Joanna Aizenberg.

The new coating builds on an award-winning technology that Aizenberg and her team pioneered called Slippery Liquid-Infused Porous Surfaces (SLIPS) - the slipperiest synthetic surface known. The new coating is equally slippery, but much more durable and fully transparent. Together these advances solve longstanding challenges in creating commercially useful materials that repel almost everything.

SLIPS was inspired by the slick strategy of the carnivorous pitcher plant, which lures insects onto the ultraslippery surface of its leaves, where they slide to their doom. Unlike earlier water-repelling materials, SLIPS repels oil and sticky liquids like honey, and it resists ice formation and bacterial biofilms as well.

While SLIPS was an important advance, it was also "a proof of principle" – the first step toward a commercially valuable technology, said lead author Dr. Nicolas Vogel.

"SLIPS repels both oily and aqueous liquids but it is expensive to make and not transparent," Vogel said. The original SLIPS materials also need to be fastened somehow to existing surfaces, which is often not easy. "It would be easier to take the existing surface and treat it in a certain way to make it slippery," he explained.

Vogel, Aizenberg, and their colleagues sought to develop a coating that accomplishes this and works as SLIPS does. SLIPS's thin layer of liquid lubricant allows liquids to flow easily over the surface, much as a thin layer of water in an ice rink helps an ice skater glide.

To create a SLIPS-like coating, the researchers corral a collection of tiny spherical particles of polystyrene, the main ingredient of Styrofoam, on a flat glass surface like a collection of Ping-Pong balls. They pour liquid glass on them until the balls are more than half buried in glass. After the glass solidifies, they burn away the beads, leaving a network of craters that resembles a honeycomb. They then coat that honeycomb with the same liquid lubricant used in SLIPS to create a tough but slippery coating.

"The honeycomb structure is what confers the mechanical stability to the new coating," said Aizenberg.

By adjusting the width of the honeycomb cells to make them much smaller in diameter than the wavelength of visible light, the researchers kept the coating from reflecting light. This made a glass slide with the coating completely transparent.